Population based studies have shown increased cardiovascular mortality and morbidity shown subclinical changes in electrocardiograms (EKG) after instillation of resuspended particles, and on inhalation of resuspended particles or of concentrated ambient particles. Patients with pre-existing cardiovascular disease are particularly at risk for acute response to particulate air pollution episodes. Implantable cardioverter defibrillators (ICD) devices monitor and identify heart arrhythmias and initiate therapeutic interventions when arrhythmias exceed predefined thresholds. The date and time of these events are stored, along with electrocardiographic recordings of the events triggering therapeutic interventions. We will abstract these data for Boston area patients to produce indices of the number and types of arrhythmias by date. Counts of arrhythmic events will then be compared to particulate and gaseous air pollution and other environmental data collected at community based ambient monitoring sites. Characteristics of ambient particulate air pollution will include mass concentrations of PM10 and PM2.5, sulfate, nitrate and elemental concentrations. Concentrations of gaseous pollutants including carbon monoxide, nitrogen dioxide, ozone, and sulfur dioxide will be measured and considered as alternative predictors of cardiac arrhythmias. Meteorological factors including temperature, humidity, and barometric pressure will be considered as independent predictors of arrhythmias, and as confounders of the air pollution associations. This study will provide quantitative exposure-response functions relating specific particle characteristics to the risk of cardiac arrhythmias, a precursor of acute cardiovascular failure and sudden death.